The symlink would mean old URI’s would still work, but any new URI’s generated (on every new page) would point to the preferred address.

I think it’s maybe also worth adding this to robots.txt a:

User-agent: *

Disallow: /your-old-folder

So that any robots that discover one of the older URLs don’t go indexing what they might consider as duplicate pages.

Longer term you could also add a 301 redirect to htaccess to ensure any links to the old content have rank properly attributed to the new location… but this is probably getting waaay too complex if it’s a new installation.

Another alternative, of course, is to just rename the folder and change your config.php – it’s more maverick than the symlink approach, but if your forum is new and has no fully qualified self referencing links contained within, then it’s not much of a wrench to just move the whole kaboodle.